Commercially available pipes have, in part, wide tolerances and considerable ovalness. This requires the internal diameter of the sleeve to be designed to be very large. Also, in order to be easily installable, additional large thermal extensions or strains occur in electrically welded sleeves having a large diameter, resulting in pressure degradation in the welding zone.
A welding sleeve is disclosed in EP-A1 022 2287 in which reinforcement from a material having a lower coefficient of thermal expansion than the sleeve member of a polyolefin helps save welding sleeve material in the sleeve body. The welding pressure required for a satisfactory weld is obtained by a smaller elongation or extension of the reinforcement during the welding process.
The coefficients of thermal expansion, however, vary between the sleeve member and the reinforcement according to differing environmental temperatures during the course of welding, resulting in differing prestresses. For example, one obtains a different welding pressure at +45.degree. C. than at -10.degree. C. Constant operability therefore cannot be assured. The reinforcement configured as a pipe-shaped member or as a wire winding or coil is additionally not resistant against corrosion and does not therefore provide for the lasting reinforcement of the welding sleeve acting also against the medium pressure.
To overcome these and other disadvantages with previous welding sleeves, it is an object of the invention to provide an economical welding sleeve which permits a constant operability at varying environmental temperatures during welding.
Another object of the invention is to provide a welding sleeve usable for larger pipe diameters exceeding, for example, 160 mm and which is not prone to corrosion.
Yet another object of the invention is to provide a method of manufacturing a welding sleeve in a simple, automatic method.